Melt the butter in a saucepan and add the flour.
Cook, stirring, for 2 minutes, then gradually stir in the milk.
Bring to the boil and simmer, still stirring, until thickened.
Add the cheese and stir until it melts and the sauce is smooth.
Use a thin pouring sauce as a basis for soups; a pouring sauce with pasta or vegetables; a coating sauce to cover fish, eggs, meat, vegetables, etc.; a very thick sauce as a basis for souffles.